Oracle - Partition by clause

Partition by is used while writing the analytical functions in Oracle.

Referring to this blog - http://www.orafaq.com/node/55

It is true that whatever an analytic function does can be done by native SQL, with join and sub-queries. But the same routine done by analytic function is always faster, or at least as fast, when compared to native SQL.


The general syntax of analytic function is:
Function(arg1,..., argn) OVER ( [PARTITION BY <...>] [ORDER BY <....>] [] )
is like "ROW " or "RANK


How are analytic functions different from group or aggregate functions?

SELECT deptno,
COUNT(*) DEPT_COUNT
FROM emp
WHERE deptno IN (20, 30)
GROUP BY deptno;

DEPTNO                 DEPT_COUNT             
---------------------- ---------------------- 
20                     5                      
30                     6                      

2 rows selected
Query-1
Consider the Query-1 and its result. Query-1 returns departments and their employee count. Most importantly it groups the records into departments in accordance with the GROUP BY clause. As such any non-"group by" column is not allowed in the select clause.
SELECT empno, deptno, 
COUNT(*) OVER (PARTITION BY 
deptno) DEPT_COUNT
FROM emp
WHERE deptno IN (20, 30);

     EMPNO     DEPTNO DEPT_COUNT
---------- ---------- ----------
      7369         20          5
      7566         20          5
      7788         20          5
      7902         20          5
      7876         20          5
      7499         30          6
      7900         30          6
      7844         30          6
      7698         30          6
      7654         30          6
      7521         30          6

11 rows selected.
Query-2
Now consider the analytic function query (Query-2) and its result. Note the repeating values of DEPT_COUNT column.
This brings out the main difference between aggregate and analytic functions. Though analytic functions give aggregate result they do not group the result set. They return the group value multiple times with each record. As such any other non-"group by" column or expression can be present in the select clause, for example, the column EMPNO in Query-2.
Analytic functions are computed after all joins, WHERE clause, GROUP BY and HAVING are computed on the query. The main ORDER BY clause of the query operates after the analytic functions. So analytic functions can only appear in the select list and in the main ORDER BY clause of the query.
In absence of any PARTITION or inside the OVER( ) portion, the function acts on entire record set returned by the where clause. Note the results of Query-3 and compare it with the result of aggregate function query Query-4.
SELECT empno, deptno, 
COUNT(*) OVER ( ) CNT
FROM emp
WHERE deptno IN (10, 20)
ORDER BY 2, 1;

     EMPNO     DEPTNO        CNT
---------- ---------- ----------
      7782         10          8
      7839         10          8
      7934         10          8
      7369         20          8
      7566         20          8
      7788         20          8
      7876         20          8
      7902         20          8
Query-3
SELECT COUNT(*) FROM emp
WHERE deptno IN (10, 20);

  COUNT(*)
----------
         8
Query-4

How to break the result set in groups or partitions?

It might be obvious from the previous example that the clause PARTITION BY is used to break the result set into groups. PARTITION BY can take any non-analytic SQL expression.
Some functions support the inside the partition to further limit the records they act on. In the absence of any analytic functions are computed on all the records of the partition clause.
The functions SUM, COUNT, AVG, MIN, MAX are the common analytic functions the result of which does not depend on the order of the records.
Functions like LEAD, LAG, RANK, DENSE_RANK, ROW_NUMBER, FIRST, FIRST VALUE, LAST, LAST VALUE depends on order of records. In the next example we will see how to specify that.

How to specify the order of the records in the partition?

The answer is simple, by the "ORDER BY" clause inside the OVER( ) clause. This is different from the ORDER BY clause of the main query which comes after WHERE. In this section we go ahead and introduce each of the very useful functions LEAD, LAG, RANK, DENSE_RANK, ROW_NUMBER, FIRST, FIRST VALUE, LAST, LAST VALUE and show how each depend on the order of the record.
The general syntax of specifying the ORDER BY clause in analytic function is:
ORDER BY [ASC or DESC] NULLS [FIRST or LAST]
The syntax is self-explanatory.

ROW_NUMBER, RANK and DENSE_RANK

All the above three functions assign integer values to the rows depending on their order. That is the reason of clubbing them together.
ROW_NUMBER( ) gives a running serial number to a partition of records. It is very useful in reporting, especially in places where different partitions have their own serial numbers. In Query-5, the function ROW_NUMBER( ) is used to give separate sets of running serial to employees of departments 10 and 20 based on their HIREDATE.
SELECT empno, deptno, hiredate,
ROW_NUMBER( ) OVER (PARTITION BY
deptno ORDER BY hiredate
NULLS LAST) SRLNO
FROM emp
WHERE deptno IN (10, 20)
ORDER BY deptno, SRLNO;

EMPNO  DEPTNO HIREDATE       SRLNO
------ ------- --------- ----------
  7782      10 09-JUN-81          1
  7839      10 17-NOV-81          2
  7934      10 23-JAN-82          3
  7369      20 17-DEC-80          1
  7566      20 02-APR-81          2
  7902      20 03-DEC-81          3
  7788      20 09-DEC-82          4
  7876      20 12-JAN-83          5

8 rows selected.
Query-5 (ROW_NUMBER example)
RANK and DENSE_RANK both provide rank to the records based on some column value or expression. In case of a tie of 2 records at position N, RANK declares 2 positions N and skips position N+1 and gives position N+2 to the next record. While DENSE_RANK declares 2 positions N but does not skip position N+1.
Query-6 shows the usage of both RANK and DENSE_RANK. For DEPTNO 20 there are two contenders for the first position (EMPNO 7788 and 7902). Both RANK and DENSE_RANK declares them as joint toppers. RANK skips the next value that is 2 and next employee EMPNO 7566 is given the position 3. For DENSE_RANK there are no such gaps.
SELECT empno, deptno, sal,
RANK() OVER (PARTITION BY deptno
ORDER BY sal DESC NULLS LAST) RANK,
DENSE_RANK() OVER (PARTITION BY
deptno ORDER BY sal DESC NULLS
LAST) DENSE_RANK
FROM emp
WHERE deptno IN (10, 20)
ORDER BY 2, RANK;

EMPNO  DEPTNO   SAL  RANK DENSE_RANK
------ ------- ----- ----- ----------
  7839      10  5000     1          1
  7782      10  2450     2          2
  7934      10  1300     3          3
  7788      20  3000     1          1
  7902      20  3000     1          1
  7566      20  2975     3          2
  7876      20  1100     4          3
  7369      20   800     5          4

8 rows selected.
Query-6 (RANK and DENSE_RANK example)

LEAD and LAG

LEAD has the ability to compute an expression on the next rows (rows which are going to come after the current row) and return the value to the current row. The general syntax of LEAD is shown below:
LEAD (, , ) OVER ()
is the expression to compute from the leading row.
is the index of the leading row relative to the current row.
is a positive integer with default 1.
is the value to return if the points to a row outside the partition range.

The syntax of LAG is similar except that the offset for LAG goes into the previous rows.
Query-7 and its result show simple usage of LAG and LEAD function.
SELECT deptno, empno, sal,
LEAD(sal, 1, 0) OVER (PARTITION BY dept ORDER BY sal DESC NULLS LAST) NEXT_LOWER_SAL,
LAG(sal, 1, 0) OVER (PARTITION BY dept ORDER BY sal DESC NULLS LAST) PREV_HIGHER_SAL
FROM emp
WHERE deptno IN (10, 20)
ORDER BY deptno, sal DESC;

 DEPTNO  EMPNO   SAL NEXT_LOWER_SAL PREV_HIGHER_SAL
------- ------ ----- -------------- ---------------
     10   7839  5000           2450               0
     10   7782  2450           1300            5000
     10   7934  1300              0            2450
     20   7788  3000           3000               0
     20   7902  3000           2975            3000
     20   7566  2975           1100            3000
     20   7876  1100            800            2975
     20   7369   800              0            1100

8 rows selected.
Query-7 (LEAD and LAG)

FIRST VALUE and LAST VALUE function

The general syntax is:
FIRST_VALUE() OVER ()
The FIRST_VALUE analytic function picks the first record from the partition after doing the ORDER BY. The is computed on the columns of this first record and results are returned. The LAST_VALUE function is used in similar context except that it acts on the last record of the partition.
-- How many days after the first hire of each department were the next
-- employees hired?

SELECT empno, deptno, hiredate ? FIRST_VALUE(hiredate)
OVER (PARTITION BY deptno ORDER BY hiredate) DAY_GAP
FROM emp
WHERE deptno IN (20, 30)
ORDER BY deptno, DAY_GAP;

     EMPNO     DEPTNO    DAY_GAP
---------- ---------- ----------
      7369         20          0
      7566         20        106
      7902         20        351
      7788         20        722
      7876         20        756
      7499         30          0
      7521         30          2
      7698         30         70
      7844         30        200
      7654         30        220
      7900         30        286

11 rows selected.
Query-8 (FIRST_VALUE)

FIRST and LAST function

The FIRST function (or more properly KEEP FIRST function) is used in a very special situation. Suppose we rank a group of record and found several records in the first rank. Now we want to apply an aggregate function on the records of the first rank. KEEP FIRST enables that.
The general syntax is:
Function( ) KEEP (DENSE_RANK FIRST ORDER BY ) OVER ()
Please note that FIRST and LAST are the only functions that deviate from the general syntax of analytic functions. They do not have the ORDER BY inside the OVER clause. Neither do they support any clause. The ranking done in FIRST and LAST is always DENSE_RANK. The query below shows the usage of FIRST function. The LAST function is used in similar context to perform computations on last ranked records.
-- How each employee's salary compare with the average salary of the first
-- year hires of their department?

SELECT empno, deptno, TO_CHAR(hiredate,'YYYY') HIRE_YR, sal,
TRUNC(
AVG(sal) KEEP (DENSE_RANK FIRST
ORDER BY TO_CHAR(hiredate,'YYYY') )
OVER (PARTITION BY deptno)
     ) AVG_SAL_YR1_HIRE
FROM emp
WHERE deptno IN (20, 10)
ORDER BY deptno, empno, HIRE_YR;

     EMPNO     DEPTNO HIRE        SAL AVG_SAL_YR1_HIRE
---------- ---------- ---- ---------- ----------------
      7782         10 1981       2450             3725
      7839         10 1981       5000             3725
      7934         10 1982       1300             3725
      7369         20 1980        800              800
      7566         20 1981       2975              800
      7788         20 1982       3000              800
      7876         20 1983       1100              800
      7902         20 1981       3000              800

8 rows selected.
Query-9 (KEEP FIRST)

How to specify the Window clause (ROW type or RANGE type windows)?

Some analytic functions (AVG, COUNT, FIRST_VALUE, LAST_VALUE, MAX, MIN and SUM among the ones we discussed) can take a window clause to further sub-partition the result and apply the analytic function. An important feature of the windowing clause is that it is dynamic in nature.
The general syntax of the is

[ROW or RANGE] BETWEEN AND

can be any one of the following

  1. UNBOUNDED PECEDING
  2. CURRENT ROW
  3. <sql_expr> PRECEDING or FOLLOWING.
    can be any one of the following
  1. UNBOUNDED FOLLOWING or
  2. CURRENT ROW or
  3. <sql_expr> PRECEDING or FOLLOWING.
For ROW type windows the definition is in terms of row numbers before or after the current row. So for ROW type windows must evaluate to a positive integer.
For RANGE type windows the definition is in terms of values before or after the current ORDER. We will take this up in details latter.
The ROW or RANGE window cannot appear together in one OVER clause. The window clause is defined in terms of the current row. But may or may not include the current row. The start point of the window and the end point of the window can finish before the current row or after the current row. Only start point cannot come after the end point of the window. In case any point of the window is undefined the default is UNBOUNDED PRECEDING for and UNBOUNDED FOLLOWING for .
If the end point is the current row, syntax only in terms of the start point can be can be
[ROW or RANGE] [ PRECEDING or UNBOUNDED PRECEDING ]
[ROW or RANGE] CURRENT ROW is also allowed but this is redundant. In this case the function behaves as a single-row function and acts only on the current row.

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